The present invention relates to an apparatus and a process for regulating ink feed controls in an offset printing machine by photoelectric measurement of printed products and by the determination of setting values for the ink feed elements from such measurement, and to an offset printing machine equipped with an appropriate apparatus for regulating ink feed controls.
The evaluation of print quality and the regulation of ink feed are usually effected by means of standardized color control strips. These control strips, printed together with the products to be printed, are evaluated densitometrically and the color values of the printing machine set accordingly. The measurement of the color control strips may take place on the printing machine while it is running by beans of so-called machine densitometers, or off-line, for example by means of an automatic scanning densitometer, wherein the control loop in both cases may be open (quality evaluation) or closed (machine regulation) in relation to the inking systems. A representative example of a computer-controlled printing machine having a closed control loop is described in U.S. Pat. Nos. 4,200,932 and 3,835,777, among others.
In actual practice, it very frequently occurs, for example for reasons of format, that the use of a color control strip is not possible. In such cases, ink feeds must be manually controlled, as before, on the basis of visual evaluation of the printed product, which manual process is highly undesirable.
It is known from U.S. Pat. No. 3,958,509, EP-Publ. No. 29561 and EP-Publ. No. 69572 that surface coverage of printing plates zone-by-zone by in-the-image measurements can be determined and evaluated to enable either manual or machine presetting of ink feed elements. This, however, merely involves a single presetting, while no ongoing regulation proper of the ink feed elements takes place during the course of printing.
A similar system, wherein a printed reference product found to be satisfactory is compared with the printed product to be evaluated by image elements in keeping with various criteria, is also known from published UK application No. 2 115 145. In the final analysis, this system leads merely to a binary quality evaluation of "good" or "bad" and is not intended, nor is it suitable, for the automatic control of ink feeds.